Web Page Design: History of HTML

Some Background

Hypertext Markup Language (HTML) is an application of a more general markup
language known as Standard Generalized Markup Language (SGML). Every HTML
document should start with an SGML declaration that gives it's definition. This
might not make a lot of sense to you right now - don't worry, you don't need
to know anything about SGML to write HTML documents.

It wasn't always that way, and probably most of the Web user agents, like
Netscape Navigator and Internet Explorer, do not use SGML to decode the HTML
markup; however, it is the intent of the World Wide Web Consortium [W3C]
(www.w3.org) which is responsible for the development of Internet Web Technologies
to encourage this view of document markup.